In data communications using packets (packet communications) such as U-Plane transmission, for example, a maximum size of a packet that can be transmitted at a time, for example, a Maximum Transmission Unit (MTU), is determined with respect to each transmission path for conveying information. When a packet with a size exceeding the MTU is received, a communication apparatus (node) on the transmission path between transmitting and receiving terminals performs fragmentation, if possible, so that the IP packet may be segmented into pieces with a size smaller than the MTU. If the packet is unable to be segmented, the fragmentation is not executed and the packet is discarded. At this time, the MTU value is notified, along with a destination unreachable message, by the ICMP (Internet Control Message Protocol), which is a protocol for forwarding IP error messages and control messages. The MTU varies in response to change in the network configuration of transmission paths attributable, for example, to handover of mobile stations during wireless communication.
If packets large in size occupy the transmission path during the packet communication, packet transmission which needs real-time performance, such as VoIP (Voice over IP), becomes unstable, possibly failing to ensure the specified transmission quality. The reason is that, where the fragmentation is executed on the network, the processing load increases, lowering the throughput of the U-Plane. To avoid this, it may be necessary that the size of packets transmitted over the network is reduced to lessen instability of the packet transmission.
Reducing the packet size, however, leads to increase of control signals and thus degradation of the transmission efficiency, as well as to increase of packet reassembly processes. Accordingly, the size of packets to be transmitted needs to be appropriately set.
Meanwhile, techniques have been known in which the MTU of a transmission path is acquired so that a terminal may transmit data packets with a size not exceeding the MTU of the transmission path (see, e.g., Japanese Laid-open Patent Publications No. 2005-204001, No. 2001-251353, No. 2004-336748, and No. 2006-157544).
However, if the transmission path MTU is acquired every time a call connection is requested from the terminal, a longer time is taken to establish the call connection.
Also, where such MTU discovery is executed by multiple terminals, the load on apparatuses constituting the transmission path increases due to the execution of the MTU discovery, giving rise to a problem that the usability of the transmission path lowers.